Method for determining sulfur as sulfur dioxide

ABSTRACT

NITROGEN INTERFERENCES TO THE IODOMETRIC AND/OR MICROCOULOMETERIC METHODS OF SULFUR DETERMINATION ARE OVERCOME BY ADDING SODIUM AZIDE TO THE TITRATION CELL ELECTROLYTE. THE IMPROVED METHOD IS APPLICABLE TO OILS, CRUDES, SOLIDS, AND PETROCHEMICALS.

United States Patent Olfice 3,585,000 Patented June 15, 1971 3,585,000 METHOD FOR DETERMINING SULFUR AS SULFUR DIOXIDE Elmars Bremanis, Park Ridge, Ill., assignor tnFiUniversal Oil Products Company, Des Plaines, Ill. No Drawing. Filed Sept. 19, 1968, Ser. No. 760,977

Int. Cl. B01k 1/00; G01n 27/42, 31/16 US. Cl. 23-430 2 Claims ABSTRACT OF THE DISCLOSURE J2 Nitrogen interferences to the iodometric and/or microcoulometeric methods of sulfur determination are overcome by adding sodium azide to the titration cell electrolyte. The improved method is applicable to oils, crudes, solids, and petrochemicals.

BACKGROUND OF THE INVENTION The presence of nitrogen in excess of 0.001 weight percent interferes in the iodometric determination of sulfur in petroleum and chemical products using the classical iodometric ASTM Methods of Test for Sulfur in Petroleum Products (High Temperature Method) (D1552-64) or its equivalent. The extent of such nitrogen interference is dependent on the type of nitrogen compound as well as the amount present. The interference results from oxidation of nitrogen to oxides of nitrogen which reacts with potassium iodide in the iodometric titration to liberate iodine. Sulfur is oxidized in the method to sulfur dioxide which is measured by the consumption of iodine in the titration. Therefore, the effect of the presence of nitrogen is to cause low, or even negative results. Since nitrogen is commonly associated with sulfur in petrolum products, petrochemicals, and chemicals, this interference constitutes a very severe limitation of the method.

Therefore, it is one object of this invention to provide an improved method for determining sulfur as sulfur dioxide in petroleum and chemical products. It is another object of this invention to provide a method of overcoming nitrogen compound interferences in the classical methods of sulfur determination so that the inherent limitations of the old methods are eliminated.

In one embodiment, our invention provides an improvement in the method for determining sulfur as sulfur dioxide in peroleum, petrochemical, and chemical products utilizing the iodometric or microcoulometric methods of sulfur determination wherein a titration cell containing an iodine-containing titration solvent is utilized, said improvement comprising overcoming nitrogen interferences in said method by adding an alkali metal azide to the titration solvent thereby preferentially reacting the nitrogen compounds without reacting the iodine.

SUMMARY OF THE INVENTION AND EXAMPLE In order to overcome the interference from nitrogen, several scrubbers and pretreatments were evaluated. From this experimentation, it was found that an alkali metal azide, and preferably sodium azide, added directly to a titration vessel, appeared to react rapidly and preferentially with the nitrogen compounds. The presence of sodium azide did not appear to otherwise participate in or interfere with the iodometric determination of sulfur as sulfur dioxide. To check these preliminary observations, several standards were analyzed with and without the presence of sodium azide in the titration vessels of the volumetrictype sulfur determinator, and in the coulometric-type sulfur analyzer as described in Adams et al., Improved Sulfur-Reacting Microcoulometric Cell for Gas Chromatography, Analytical Chemistry, vol. 38, 196 6, p. 1094.

An induction furnace and automatic volumetric sulfur apparatus was used where the test method called for a volumetric-type sulfur determinator. A microcoulometer with a 300P cell was used for iodine generation where the method called for a coulometric-type sulfur analyzer. The combustion train used with the microcoulometer employed a 13 mm. outside diameter, 10 mm. inside diameter, 115- cm. quartz tube filled with quartz chips, coarsely ground to pass a inch screen. The quartz tube was maintained at 1150 C. using two heavy duty combustion furnaces in series. Oxygen carrier flow rates from 20 to 300 mL/min. were preferred, although a flow of ml./min. was normally used with this apparatus. Sodium azide (practical) was used as the reagent.

For the coulometric analyses, 3 to 5 mg. of solid sodium azide was added to the titration cell solvent. In order to equilibrate the system prior to the first determination, a sample of S0 gas was injected or a sample of the product was burned. A fresh portion of sodium azide was added whenever the cell solvent was changed.

For the volumetric-type determinations, 500 to 700 mg. of solid sodium azide was added to the titration cell solvent prior to each determination. No other changes in the normal operating procedures for the method were required for either approach to the iodometric sulfur analysis. To determine the efficacy of sodium azide addition in overcoming nitrogen interference, several standards and samples were analyzed with and without added sodium azide. The following data comparing both the coulometric and volumetric methods were obtained:

TABLE 1.ANALYSIS OF STANDARDS AND SAMPLES WITH AND WITHOUT ADDED SODIUM AZIDE Percent sulfur found Percent present No NaNz added NaNa added Sample N S Volumetric Coulometric Volumetric Coulometrio Black 2 Negative 20 29, 20. 53

2. 07, 2. 08 2. 13, 2. 12 2. 16, 2. 12 1. 11, 1. 07 1. 06, 1. 12 5. 08 4. 92 5. 43 5. 67 4. 38 4. 48 0.12 0.1200, 0.1207, 0. 1228, 0. 1196, 0. 1202 Heavy oil, Sample 1 1. 25 1. 36, 1. 34 Heavy oil, Sample 2 5, .1533, 0. 1591, 0.17, 0.15, 0.13, 0.1521, 0. 1547, 0. 16, 0.15 0.1573 Solid catalyst, spent 2. 44 3. 31, 3. 31 Crude oil 2 216, 2.1 2.16 2. 18, 217

1 Generates iodine; gives black solution in the volumetric method and a negative response in the case of the coulometrie method it much nitrogen is present.

2 Oxygen bomb analysis, ASTM Method of Test for Sulfur in Petroleum Products by the Bomb Method (D129*64).

Therefore, it can be concluded that an alkali metal azide, and prefereably sodium azide added to an iodinecontaining titration solvent is eflective in overcoming the interference from nitrogen where it is encountered in the iodometric determination of sulfur as sulfur dioxide. For ASTM Method D1552 modified by the improvement of my invention, the relative standard deviation was 2% for the l to 20% sulfur level. For the coulometric sulfur method, the relative error in the presence of about 8% nitrogen was about 1.0% and the relative standard deviation was 1.1%. As shown above, samples analyzed included oils, crudes, and solids and included sulfur levels ranging from about 0.1 to about 20%. The coulometric method is more sensitive and has better precision than the volumetric method although the two methods are comparable in accuracy, even in the presence of large amounts of nitrogen compounds provided the improvement of my invention, namely, the addition of an alkali metal azide, such as sodium azide, to the titration solvent is utilized.

I claim:

1. In a method for determining sulfur as sulfur dioxide in petroleum, petrochemical, or chemical products which contain sulfur and nitrogen utilizing the iodometric or References Cited UNITED STATES PATENTS 2/ 1954 Halvorson et al. 23-230PC OTHER REFERENCES Pardue et al., Chem. Abstr. 58, 7l25d (1963).

MORRIS O. WOLK, Primary Examiner R. E. REESE, Assistant Examiner US. Cl. X.R. 204-1 

